1. Field of the Invention
The present invention relates to methods and apparatus for separating, cooling and liquefying component gases from each other in a pressurized mixed gas stream. More particularly, the invention is directed to separation techniques that utilizes some of the components of the mixed gas stream that have already been separated to cool portions of the mixed gas stream that subsequently pass through the apparatus.
2. Description of the Prior Art
Individual purified gases, such as oxygen, nitrogen, helium, propane, butane, methane, and many other hydrocarbon gases, are used extensively throughout many different industries. Such gases, however, are typically not naturally found in their isolated or purified state. Rather, each individual gas must be separated or removed from mixtures of gages For example, purified oxygen is typically obtained from the surrounding air which also includes nitrogen, carbon dioxide and many other trace elements. Similarly, hydrocarbon gases such as ethane, butane, propane, and methane are separated from natural gas which is produced from gas wells, landfills, city sewage digesters, coal mines, etc.
In addition to separating or purifying the individual gases, it is often necessary to liquify gases. For example, liquefied natural gas (LNG), which is primarily methane, is used extensively as an alternative fuel for operating automobiles and other machinery. The natural gas must be liquified or compressed since storing natural gas in an uncompressed vapor or gas state would require a storage tank of unreasonably immense proportions. Condensing or liquifying other gases is also desirable for more convenient storage and/or transportation.
The liquefaction of gases can be accomplished in a variety of different ways. The fundamental method is to compress the gas and then cool the compressed gas by passing it through a number of consecutively colder heat exchanges. A heat exchanger is simply an apparatus or process wherein the gas or fluid to be cooled is exposed to a colder environment which draws heat or energy from the gas or fluid, thereby cooling the gas. Once a gas reaches a sufficiently low temperature for a set pressure, the gas converts to a liquid.
The cold environment needed for each heat exchanger is generally produced by an independent refrigeration cycle. A refrigeration cycle, such as that used on a conventional refrigerator, utilizes a closed loop circuit having a compressor and an expansion valve. Flowing within the closed loop is a refrigerant such as Freon(copyright). Initially, the refrigerant is compressed by the compressor which increases the temperature of the refrigerant. The compressed gas is then cooled. This is often accomplished by passing the gas through air or water cooled coils. As the compressed gas cools, it changes to a liquid. Next, the liquid passes through an expander valve which reduces the pressure on the liquid. This pressure drop produces an expansion of the liquid which may vaporize at least a portion thereof and which also significantly cools the now combined liquid and gas stream.
This cooled refrigerant stream now flows into the heat exchanger where it is exposed to the main gas stream desired to be cooled. In this environment, the refrigerant stream draws heat from the main stream, thereby simultaneously cooling the main stream and warming the refrigerant stream. As a result of the refrigerant being warmed, the remaining liquid is vaporized to a gas. This gas then returns to the compressor where the process is repeated.
By passing the main gas stream through consecutive heat exchanges having lower and lower temperatures, the main stream can eventually be cooled to a sufficiently low temperature that it converts to a liquid. The liquid is then stored in a pressurized tank.
Although the above process has been useful in obtaining liquefied gasses, it has several shortcomings. For example, as a result of the process using several discrete refrigeration cycles, each with its own compressor, the system is expensive to build, costly to run and maintain, and has an overall high complexity. A significant cost for any closed loop refrigeration system is the purchase and operation of the compressor. Not only does the compressor represent the process"" largest capital expenditure, it also represents a major problem in the process system""s flexibility. Once a compressor size is chosen, the process can only handle mass flow rates capable of being adequately compressed by the chosen compressor. In order to have wide flexibility in process flows, multiple compressors are then needed. These additional compressors also add to the cost and risk of equipment failure.
To make conventional systems cost effective to operate, such systems are typically built on a large scale. As a result, fewer facilities are built making it harder to get gas to the facility and to distribute liquified gas from the facility. By their vary nature, large facilities are required to store large quantities of liquefied gas prior to transport. Storage of LNG can be problematic in that once the LNG begins to warm from the surrounding environment, the LNG begins to vaporize within the storage tank. To prevent pressure failure of the tank, some of the pressurized gas is permitted to vent. Such venting is not only an environmental concern but is also a waste of gas.
The steps for purification or separation of the different gases from a main mixed gas are often accomplished prior to the liquefaction process and can significantly add to the expense and complexity of the process. As a result, many productive gas wells having high concentrations of undesired gases or elements are often capped rather than processed.
Accordingly, it is an object of the present invention to provide gas processing systems which can liquify at least a portion of a mixed gas stream.
Another object of the present invention is to provide gas processing systems which simultaneously purify the liquefied gas by separating off the other mixed gases.
It is also an object of the present invention to provide the above systems that can separate off each component gas of the mixed gas in a substantially pure form for subsequent use of each of the individual gases.
Yet another object of the present invention is to provide the above system which can be operated without the required use of independently operated compressors or refrigeration systems.
Still another object of the present invention is to provide the above systems which can be effectively produced to achieve any desired flow capacity and, furthermore, can be manufactured as small mobile units that can be operated at any desired location.
To achieve the forgoing objectives, and in accordance with the invention as disclosed and broadly claimed herein, a gas processing system and method of operation is provided for separating and cooling components of a pressurized mixed gas stream for subsequent liquefaction of a final or remaining gas stream. This inventive system and process comprises passing a pressurized mixed gas stream through a series of repeated cycles until a final substantially purified gas stream for liquefying is achieved. Each cycle comprises: (1) cooling the pressurized mixed gas stream in a heat exchanger so as to condense one or more of the gas components having the highest condensation point; (2) separating the condensed components from the remaining mixed gas stream in a gas-liquid separator; (3) cooling the separated condensed component stream by passing it through an expander; and then (4) passing the cooled component stream back through the heat exchanger such that the cooled component streams function as the refrigerant for the heat exchanger. The component stream then exits the system for use depending on the type and temperature of gas.
The above cycle is then repeated for the remaining mixed gas stream so as to draw off the next component gas and further cool the remaining mixed gas stream. The process continues until all of the unwanted component gases are removed. The final gas stream, which in the case of natural gas will be substantially methane, is then passed through a final heat exchanger. The final cooled gas stream is then passed through an expander which decreases the pressure on the gas stream. As the pressure decreases, the stream is cooled causing a portion of the gas stream to liquify within a tank. The portion of the gas which is not liquified is passed back through each of the heat exchangers where it functions as a refrigerant.
Where the initial pressure of the mixed gas stream is sufficiently high, the inventive systems can be operated solely from the energy produced by dropping the pressure. As such, there is no need for independently powered compressors or refrigeration cycles. In one embodiment, however, the final expander can comprise a turbo expander which runs a turbine as the gas is expanded therethrough. The electrical or mechanical energy from the turbine can be used to input energy into the system at any desired location. For example, the turbo expander can run a compressor which is used to increase the pressure of the initial gas stream. Where there is insufficient pressure in the initial gas stream, which cannot be sufficiently increased by the turbo expander, the present invention also envisions that an independently operated compressor can be incorporated into the system.
The inventive system has a variety of benefits over conventional systems. For example, by not needing independently operated compressors or refrigeration systems, the inventive system is simpler and less expensive. Furthermore, the inventive system can be effectively constructed to fit any desired flow parameters at virtually any location. For example, one unique embodiment of the present invention is to incorporate the inventive system onto a movable platform such as a trailer. The movable unit can then be positioned at locations such as a well head, factory, refueling station, or distribution facility.
An additional benefit of the present invention is that the system and process can be used to separate off purified component gas streams while simultaneously purifying the final gas stream. For example, during the production of LNG, the system can be designed, depending on the gas composition, to condense off substantially pure propane, butane, ethane, and any other gases present for subsequent independent use in their corresponding markets. By removing all the component gases, the final methane gas is also substantially purified. Accordingly, the inventive system and process can also be used to effectively operate gas wells that have historically been caped for having too high of a concentration of undesired components.